In known pressure control valves the monitored pressure acts against a piston or diaphragm in opposition to a fixed force, such as a spring or jacket pressure. A poppet valve carried by the diaphragm may be opened when the monitored pressure is overcome by the fixed force to initiate a pressure build up in the system in controlling downstream pressure, or it may be opened by overcoming the fixed force to receive a monitored upstream pressure. In either event, there is often a lag between the level of the pressure being controlled and that in the sensing chamber whereby there is often a considerable amount of cycling or chatter as the valves seek to maintain the desired level. Others have sought to cure such chatter by the provision of mechanical means such as dash pots, friction restrainers and the like, or by the provision of an orifice in the sensing port whereby the operation of the valve itself is dampened. However, if the orifice is too restrictive it could become clogged or occluded by a particle of matter entrained in the gas stream and, should such occlusion operate to prevent entry of the sensing fluid, the valve could be held in open position (or held closed in the case of upstream monitoring) causing an undesirable and, perhaps, dangerous pressure build up in the system.